1. Field of the Invention
The present invention relates to a bearing unit in which a dynamic fluid bearing adapted to receive a rotary shaft inserted thereinto is covered by a unit main body and to a motor using the bearing unit.
2. Description of the Related Art
FIG. 8 is a schematic cross-sectional view for assistance in explaining a bearing unit provided with a non-contact rotational seal (hereinafter, simply referred to as “the seal”) in related art. A bearing unit 30 is configured as below. A shaft 34 is rotatably supported by a slide bearing 31 and a thrust bearing 39 (a pivot bearing in this case). A non-contact rotational seal 30a is composed of a gap portion 37 defined between the shaft 34 and a gap-forming member 38 and tapered in cross-section axially-inwardly from the slide bearing 31 for preventing leakage of lubricating oil. A stator member 36 holds the slide bearing 31, the thrust bearing 39 and the gap-forming member 38.
Then, a description is made of how the gap portion 37 tapered axially-inwardly from the slide bearing 31 provides a non-contact rotational seal for retaining lubricating oil inside the bearing unit.
Fluid tends to move in a narrower width due to the capillary phenomenon. If it is intended to retain lubricating oil inside the bearing unit, then the bearing unit is preferably provided with a gap that is tapered in cross-section toward the inside thereof. The lubricating oil in the gap thus tapered will move toward the inside of the bearing unit for retainment.
In this case, the drawing pressure P generated by the capillary phenomenon is represented by the equation, P=2γ cos θ/r, where γ is surface tension occurring between lubricating oil and a lubricating oil-contact object (if this object is the shaft 34, it is made of stainless steel or duralumin; if the object is the gap-forming member 38, it is made of metal or resin), θ is a contact angle between the lubricating oil and the lubricating oil-contact object and r is a width of the gap portion 37.
That is to say, P∝1/r; therefore, the lubricating oil is drawn toward the inside of the bearing unit having a smaller gap width r for retainment.
In this way, the non-contact rotational seal 30a in the related art can seal the lubricating oil of the rotational portion in a non-contact manner by devising the shape of the seal portion and using the surface tension of the lubricating oil. Because of the non-contact seal, the non-contact rotational seal 30a is of an excellent lubricating oil sealing system that has no torque loss and maintains excellent rotational-mechanical accuracy such as no axial vibration.
FIG. 9 is a cross-sectional view of a seal portion extracted from the bearing unit 40 illustrated in Japanese Patent Laid-open No. 2005-127514 (hereinafter referred as Patent Document 1). The seal portion of the bearing unit 40 intends to solve the difficulty of thinning which is a defect of the bearing unit 30 shown in FIG. 8. That is to say, a gap portion 47 defined by the end face of a slide bearing 41 and a gap-forming member 48 is tapered in cross-section toward the shaft 44.
FIG. 10 is a cross-sectional view of a seal portion extracted from the bearing unit 50 illustrated in Japanese Patent Laid-open No. 2004-36892 (hereinafter referred as Patent Document 2). The seal portion of the bearing unit 50 is provided on the end face portion of a slide bearing 51 with a gap portion 57 which is defined by a plate-like member 58b and a gap-forming member 58a so as to be tapered in cross-section toward the outside.